The DS's touchscreen is pretty scratch resistant, but it CAN get scratched over time. I would put a protector on it just to be safe. It's better to scratch a protector and be able to change it than to scratch the touchscreen directly and be stuck with it that way.
When you buy protectors, they come with a top screen protector also. I would consider the top one as just optional as to whether you put it on the DS or not. How many times do you really touch the top screen anyway? Possibly, you could cut it down to fit the bottom screen as a replacement for if you ever scratch up the regular bottom screen protector. (the top screen protector is larger in size)
If you do buy some protectors, buy quality ones.....the Hori brand are considered the best. Depending on where you get them, they're about $5-$10.